1. Field of the Invention
The present invention relates to an internal combustion engine mounted in a small planing boat that planes across the water.
2. Description of Background Art
In an internal combustion engine mounted in a small planing boat, cooling of the internal combustion engine is effected by using the water on which the small planing boat is floated as cooling water. Water is introduced from the downstream positive-pressure side of a jet propulsion pump driven by the internal combustion engine, and is supplied to desired portions of the internal combustion engine.
While cooling water is made to circulate in the cooling system of an internal combustion engine mounted in a vehicle that travels on the ground, in the cooling system of an internal combustion engine mounted in a small planing boat, new cooling water is constantly supplied to cool the internal combustion engine. Accordingly, during cold operation, supercooling may occur before the warm-up of the internal combustion engine.
When supercooling of the internal combustion engine occurs, the amount of blow by gas that blows through the gap between the piston and the cylinder increases, and so-called dilution, whereby fuel dissolves into lubricating oil to dilute the lubricating oil, proceeds to accelerate degradation of oil.
In view of this, in an internal combustion engine mounted in a small planing boat, cooling water passes through the exhaust system before being supplied to the internal combustion engine main body. See, for example, JP-A No. 2003-49645.
In the cooling system for the internal combustion engine of the water jet propulsion boat (small planing boat) disclosed in JP-A No. 2003-49645, the cooling water introduced from the water suction port of the jet propulsion unit is branched to form two cooling water paths.
One of the cooling water paths is a path leading to the cylinder and then to the cylinder head of the internal combustion engine main body via the upstream-side exhaust pipe and the exhaust manifold. The cooling water that has been raised in temperature in the upstream-side exhaust pipe and the exhaust manifold is supplied to the water jackets of the cylinder and cylinder head, thereby preventing supercooling of the internal combustion engine before warm-up to alleviate dilution of lubrication oil.
The other cooling water path is a path leading to the downstream-side exhaust pipe and then to the muffler via the oil tank. The cooling water that has cooled the oil in the oil tank is supplied to the downstream-side exhaust pipe and the muffler to thereby cool the downstream side of the exhaust pipe and the muffler.
However, once the internal combustion engine has been warmed up, in the former cooling water path for cooling the internal combustion engine main body, the temperature of the cooling water that has passed through the upstream-side exhaust pipe and the exhaust manifold is too high. Since such high-temperature cooling water is supplied to the water jackets of the cylinder and cylinder head, the internal combustion engine cannot be cooled efficiently.